The present invention relates to a radio pager and, more particularly, to a radio pager capable of receiving a paging signal subjected to multilevel digital frequency modulation.
In a radio pager of the type described, a paging signal coming in through an antenna is applied to a radio or radio frequency (RF) section. The RF section amplifies the paging signal and converts its frequency to thereby output an intermediate frequency (IF) signal. The IF signal is applied to a demodulator. The demodulated voltage, or analog signal, is applied to an analog-to-digital converter (ADC). The ADC converts the input voltage to a digital on the basis of first and second reference voltages which respectively define the upper limit and lower limit of the conversion range assigned to the analog signal or demodulated voltage. The digital signal output from the ADC is delivered to a controller including a decoder. The decoder decodes the input digital signal and outputs the resulting address number, message and so forth. The controller compares the decoded address number and an address number assigned to the pager and stored in an address memory beforehand. If the two address numbers compare equal, the controller alerts the user of the pager to the incoming call.
It is optimal that the reference voltages delimiting the signal conversion range of the ADC be matched to the maximum variation range of the demodulated voltage. However, when the demodulation characteristic of the demodulator noticeably depends on temperature, it is necessary to broaden the signal conversion range of the ADC. A broader ADC conversion range directly translates into a lower ADC resolution and therefore into a greater ADC error. As a result, the output digital signal is more susceptible to noise and other undesirable factors. Further, the relation between the signal conversion range of the ADC and the demodulated voltage, e.g., its center voltage depends on temperature. It is therefore likely that despite that the demodulator receives IF signals identical in content or data, the ADC outputs digital signals different in content.